The use of spam to send advertisements to email users is becoming increasingly popular. Like its paper-based counterpart—junk mail, receiving spam is annoying.
Therefore, considerable effort is being brought to bear on the problem of filtering spam before it reaches the in-box of a user.
Currently, rule-based filtering systems that use rules written by a human to filter spam are available. As examples of the rules, consider the following rules:                (a) “if the subject line has the phrase “make money fast” then mark as spam;” and        (b) “if the from field is blank, then mark as spam.”        
Usually thousands of such specialized rules are necessary in order for a rule-based filtering system to be effective in filtering spam. Each of these rules are written by a human, which adds to the cost of rule-based filtering systems.
Another problem is that senders of spam (spammers) are adept at changing spam to render the rules ineffective. For example consider the rule (a), above. A spammer will observe that spam with the subject line “make money fast” is being blocked and could, for example, change the subject line of the spam to read “make money quickly.” This change in the subject line renders rule (a) ineffective. Thus, a human would need to write a new rule to filter spam with the subject line “make money quickly.” In addition, the old rule (a) will still have to be retained by the system.
With rule-based filtering systems, each incoming email message has to be checked against thousands of active rules. Therefore, rule-based filtering systems require fairly expensive hardware to support the intensive computational load of having to check each incoming electronic mail message against the thousands of active rules. Further, the human intensive nature of rule writing adds to the cost of rule-based systems. The term “email” as used herein is to be interpreted broadly to include any type of electronic message including voicemail messages, short message service (SMS) messages, multi-media messaging service (MMS) messages, facsimile messages, etc.